The present invention relates to a corrosion-inhibiting coating, process for creating the corrosion-inhibiting coating, and a corrosion-inhibiting coating bath. More specifically, the present invention relates to a coating, process, and system using zinc- or a zinc-alloy as an adherent that is directly deposited onto a steel surface for enhanced corrosion protection.
Steel or cast iron materials such as those used for fasteners, automotive bodies, and industrial processing equipment require protection from corrosion due to the low oxidation-reduction (redox) potential of iron. Typically, these materials are coated with a thin xe2x80x9csacrificialxe2x80x9d coating of a material with an even lower redox potential. The two materials that are typically used as sacrificial materials for steels are cadmium and zinc, or alloys composed of the same. During corrosive attack, these cadmium or zinc sacrificial materials are themselves preferentially corroded, maintaining the structural integrity of the underlying steel.
In instances where these sacrificial materials are removed from the steel surface, corrosive attack of the underlying steel will begin. For example, if the zinc layer which protects the steel is removed, then the underlying steel begins to corrode. Additionally, if the steel is galvanically coupled to a third metal that has a lower redox potential than iron, then that third metal will begin to corrode once the xe2x80x9csacrificialxe2x80x9d layer of zinc or cadmium is removed. This process is frequently observed during aircraft maintenance procedures. Cadmium-plated steel fasteners are used in aluminum alloy wing and fuselage sections. During routine maintenance, the cadmium plate is frequently removed from the fasteners, setting up a steel-aluminum galvanic couple. This inevitably results in corrosion of the lower redox potential material (aluminum).
A method of replacing this sacrificial layer over the steel surfaces is therefore advantageous for many applications. Zinc and zinc-containing alloys are preferred for this application, due to the toxic nature of cadmium. However, the conventional methods of applying zinc (e.g., electroplating or hot-dip processes) are not suitable for this application because neither is practical to repair the steel piece without removal of that part or steel piece. In addition, both processes require a large degree of energy expenditure in order to perform a simple repair operation. Therefore, replating an automotive or aircraft component in the field in order to replace this sacrificial layer will require a large electrical expenditure. The application of a molten zinc layer to a structure in need of repair requires a high temperature (419.5xc2x0 C.), but this high temperature may damage other structural components.
Another method involves the incorporation of zinc dust or a zinc metal-zinc oxide mixture within a polymer film (e.g., a paint), which is then applied directly onto the steel surface (i.e., zincrometal). This severely limits the successful application of conversion or phosphate coatings for subsequent paint application. In order to function properly, conversion or phosphate coatings must be applied directly onto a metal surface (e.g., zinc). Application of barrier films that contain zinc dust may result in superior corrosion protection, but the resultant adhesion to such barrier film coatings is poor.
Kimura et al. (U.S. Pat. No. 5,116,664) teaches using electroless plating where the electroless plating bath contains a metal salt, including zinc salts, and a reducing agent, including sodium hypophosphate. The electroless plating bath may also contain chelating stabilizers and buffers. However, Kimura teaches using such plating system to create a titanium-mica composite material and not for corrosion protection of steel surplus. Also, Kimura does not disclose using a fluoride preparative in his bath.
Thus, there is a need in the art for a coating which provides superior adhesion and corrosion protection for steel surfaces.
This need is met by the present invention which provides an electroless zinc coating for corrosion protection of steel surfaces. The present invention utilizes improved electroless zinc deposition techniques to achieve a tight, adherent zinc coating that is directly applied to the steel surface.
In accordance with one embodiment of the present invention, a corrosion-inhibiting coating is provided comprising a zinc source, a complexing agent for the zinc source, and a reducing agent. Generally, the zinc source is water-soluble. Generally, the zinc source is selected from zinc chloride, zinc bromide, zinc iodide, zinc sulfate, zinc chlorate, zinc nitrate, zinc perchlorate, zinc bromate, zinc acetate, zinc fluosilicate, zinc permanganate, zinc propionate, zinc citrate, zinc butyrate, zinc formate, zinc fluoride, zinc lactate, or zinc benzoate. The zinc source may have a zinc concentration greater than or equal to 1.0 M and less than or equal to the maximum solubility of the zinc source in water. Preferably, the zinc source may have a concentration from about 2.5M to about 5.0M.
The coating further comprising a preparative agent. Generally, the preparative agent is a fluoride source. Generally, the fluoride source is selected from hydrofluoric acid, ammonium fluoride, lithium fluoride, sodium fluoride, potassium fluoride, potassium bifluoride, zinc fluoride, aluminum fluoride, hexafluorozirconates, hexafluorotitanates, hexafluorosilicates, fluoroaluminates, fluoroborates, fluorophosphates, or fluoroantimonates. The preparative agent may be selected from sulfuric acid, hydrochloric acid, hydrobromic acid, hydriodic acid, phosphoric acid, phosphorous acid, boric acid, or carboxylic acid. Preferably, the preparative agent is a concentration from about 0.2M to about 0.6M.
The complexing agent may be a nitrogen-containing compound. Generally, the nitrogen-containing compound is selected from ammonium compounds, substituted ammonium, ammonia, amines, aromatic amines, porphyrins, amidines, diamidines, guanidines, diguanidines, polyguanidines, biguanides, biguanidines, imidotricarbonimidic diamides, imidotetracarbonimidic diamides, dibiguanides, bis(biguanidines), polybiguanides, poly(biguanidines), imidosulfamides, diimidosulfamides, bis(imidosulfamides), bis(diimidosulfamides), poly(imidosulfamides), poly(diimidosulfamides), phosphoramidimidic triamides, bis(phosphoramidimidic triamides), poly(phosphoramidimidic triamides), phosphoramidimidic acid, phosphorodiamidimidic acid, bis(phosphoramidimidic acid), bis(phosphorodiamidimidic acid), poly(phosphoramidimidic acid), poly(phosphorodiamidimidic acid), phosphonimidic diamides, bis(phosphonimidic diamides), poly(phosphonimidic diamides), phosphonamidimidic acid, bis(phosphonamidimidic acid), poly(phosphonamidimidic acid), azo compounds, formazan compounds, azine compounds, Schiff Bases, hydrazones, or hydramides.
The complexing agent may be a phosphorus-containing compound. Generally, the phosphorous-containing compound is selected from phosphines, aromatic phosphines, or substituted phosphonium ions (PR4+) wherein R is an alkyl, aromatic, or acyclic organic constituent of a C1 to C8. A ratio of complexing agent to the zinc source is generally from about 0.5:1 to about 4:1. Preferably, the ratio of the complexing agent to the zinc source may be from about 2:1 to about 4:1.
The reducing agent typically has a reduction potential lower than xe2x88x920.76 volts in acidic conditions. Generally, the reducing agent has a reduction potential lower than xe2x88x921.04 volts under basic conditions. Generally, the reducing agent is selected from formate, borohydride, tetraphenylborate, hypophosphite, hydroxylamine, hydroxamates, dithionite, trivalent titanium, trivalent vanadium, or divalent chromium. Preferably, the reducing agent has a concentration greater than or equal to 0.5M but less than or equal to 1.0M.
The coating may further comprise an additional metal source. Generally, the additional metal source is selected from manganese, cadmium, iron, tin, copper, nickel, indium, lead, antimony, bismuth, cobalt, or silver.
The coating may further comprise a thickening agent. The thickening agent is generally selected from starch, dextrin, gum arabic, albumin, gelatin, glue, saponin, gum mastic, gum xanthan, hydroxyalkyl celluloses, polyvinyl alcohols, polyacrylic acid and its esters, polyacrylamides, ethylene oxide polymers, polyvinylpyrrolidone, alkyl vinyl ether copolymers, colloidal suspensions of aluminum oxide or hydrated aluminum oxide, colloidal suspensions of magnesium oxide or hydroxide, or colloidal suspensions of silicon or titanium oxides. Generally, the coating comprises between about 0.1 to about 50 parts by weight per 100 parts by weight of water of a thickening agent. Preferably, the coating may comprise between about 0.1 to about 20 parts by weight per 100 parts by weight of water of a thickening agent.
In another embodiment of the present invention, a process for creating a corrosion-inhibiting coating is provided comprising the steps of preparing a first bath, preparing a second bath containing a reducing agent, providing a steel surface, depositing the first bath onto the steel surface, and then, depositing the second bath onto the steel surface. The first bath generally comprises a zinc source and a complexing agent for the zinc. The process may further comprise the step of precleaning the steel surface prior to depositing the first bath onto the steel surface. The process may further comprise masking a portion of the steel surface prior to depositing the first bath onto the steel surface. The process may further comprise the step of rinsing the steel surface after depositing the second bath onto the steel surface. The process may further comprise the step of drying the steel surface after depositing the second bath onto the steel surface. The zinc source may have a concentration greater than or equal to 1.0 M and less than or equal to the maximum solubility of the zinc source in water. Generally, the zinc source is water-soluble. Generally, the zinc source is selected from zinc chloride, zinc bromide, zinc iodide, zinc sulfate, zinc chlorate, zinc nitrate, zinc perchlorate, zinc bromate, zinc acetate, zinc fluosilicate, zinc permanganate, zinc propionate, zinc citrate, zinc butyrate, zinc formate, zinc fluoride, zinc lactate, or zinc benzoate. Preferably, the zinc source has a concentration from about 2.5M to about 5.0M.
The first bath may further comprises a preparative agent. The preparative agent is generally a fluoride source. The fluoride source is typically selected from hydrofluoric acid, ammonium fluoride, lithium fluoride, sodium fluoride, potassium fluoride, potassium bifluoride, zinc fluoride, aluminum fluoride, hexafluorozirconates, hexafluorotitanates, hexafluorosilicates, fluoroaluminates, fluoroborates, fluorophosphates, or fluoroantimonates. The preparative agent may be selected from sulfuric acid, hydrochloric acid, hydrobromic acid, hydriodic acid, phosphoric acid, phosphorous acid, boric acid, or carboxylic acid. Preferably, the preparative agent has a concentration from about 0.2M to about 0.6M. The complexing agent may be a nitrogen-containing compound. Generally, the nitrogen-containing compound is selected from an ammonium compound, substituted ammonium, ammonia, amines, aromatic amines, porphyrins, amidines, diamidines, guanidines, diguanidines, polyguanidines, biguanides, biguanidines, imidotricarbonimidic diamides, imidotetracarbonimidic diamides, dibiguanides, bis(biguanidines), polybiguanides, poly(biguanidines), imidosulfamides, diimidosulfamides, bis(imidosulfamides), bis(diimidosulfamides), poly(imidosulfamides), poly(diimidosulfamides), phosphoramidimidic triamides, bis(phosphoramidimidic triamides), poly(phosphoramidimidic triamides), phosphoramidimidic acid, phosphorodiamidimidic acid, bis(phosphoramidimidic acid), bis(phosphorodiamidimidic acid), poly(phosphoramidimidic acid), poly(phosphorodiamidimidic acid), phosphonimidic diamides, bis(phosphonimidic diamides), poly(phosphonimidic diamides), phosphonamidimidic acid, bis(phosphonamidimidic acid), poly(phosphonamidimidic acid), azo compounds, formazan compounds, azine compounds, Schiff Bases, hydrazones, or hydramides. The complexing agent may be a phosphorus-containing compound. The phosphorous-containing compound is generally selected from phosphines, aromatic phosphines, or substituted phosphonium ions (PR4+) wherein R is an alkyl, aromatic, or acyclic organic constituent of a C1 to C8. The ratio of the complexing agent to the zinc source is typically from about 0.5:1 to about 4:1. The ratio of the complexing agent to the zinc source may be from about 2:1 to about 4:1. Generally, the reducing agent has a reduction potential lower than about xe2x88x920.76 volts in acidic conditions. Generally, the reducing agent has a reduction potential lower than about xe2x88x921.04 volts under basic conditions. Generally, the reducing agent is selected from formate, borohydride, tetraphenylborate, hypophosphite, hydroxylamine, hydroxamates, dithionite, trivalent titanium, trivalent vanadium, or divalent chromium. Generally, the reducing agent has a concentration greater than or equal to 0.5M but less than or equal to 1.0M.
The first bath may further comprise an additional metal source. Generally, the additional metal source is selected from manganese, cadmium, iron, tin, copper, nickel, indium, lead, antimony, bismuth, cobalt, or silver. The first bath may further comprise a thickening agent. Generally, the thickening agent is selected from starch, dextrin, gum arabic, albumin, gelatin, glue, saponin, gum mastic, gum xanthan, hydroxyalkyl celluloses, polyvinyl alcohols, polyacrylic acid and its esters, polyacrylamides, ethylene oxide polymers, polyvinylpyrrolidone, alkyl vinyl ether copolymers, colloidal suspensions of aluminum oxide or hydrated aluminum oxide, colloidal suspensions of magnesium oxide or hydroxide, or colloidal suspensions of silicon or titanium oxides. Generally, the coating comprises between about 0.1 to about 50 parts by weight per 100 parts by weight of water of a thickening agent. Preferably, the coating may comprise between about 0.1 to about 20 parts by weight per 100 parts by weight of water of a thickening agent.
In yet another embodiment of the present invention, a process for creating a corrosion-inhibiting coating is provided comprising the steps of providing a steel surface; precleaning the steel surface; masking the steel surface; rinsing the steel surface; applying a first bath to the steel surface wherein the first bath comprises a zinc source, a preparative agent, and a complexing agent for the zinc; applying a second bath to said steel surface wherein the second bath comprises a strong reducing agent; rinsing the steel surface; and drying the steel surface.
In another embodiment of the present invention, a process for creating a corrosion-inhibiting coating is provided comprising the steps of applying a first bath to the steel surfaces wherein the first bath comprises a zinc source, a complexing agent for the zinc, and a preparative agent. The process may further include the step of applying a second bath to the steel surfaces wherein the second bath comprises a reducing agent.
In another embodiment of the present invention, a corrosion-inhibiting system is provided comprising a first bath wherein the first bath comprises a zinc source and a complexing agent for the zinc source. The system may further comprise a preparative agent. Generally, the preparative agent is a fluoride source. Generally, the fluoride is selected from the group consisting of hydrofluoric acid, ammonium fluoride, lithium fluoride, sodium fluoride, potassium fluoride, potassium bifluoride, zinc fluoride, aluminum fluoride, hexafluorozirconates, hexafluorotitanates, hexafluorosilicates, fluoroaluminates, fluoroborates, fluorophosphates, and fluoroantimonates. They system may further comprise a second bath containing a reducing agent. The reducing agent generally has a reduction potential lower than xe2x88x920.76 volts in acidic conditions. Generally, the reducing agent has a reduction potential lower than xe2x88x921.04 volts under basic conditions. The first bath may further comprise an additional metal source. The first bath may further comprise an organic thickening agent.
Accordingly, it is an object of the invention to provide a corrosion-inhibiting coating, a process for creating the corrosion-inhibiting coating, and a process for creating a corrosion-inhibiting coating bath. Other objects of the invention will become apparent in light of the description of the invention embodied herein.